Methods and apparatus for estimating a state of health of a battery

1. An apparatus for estimating a state of health of a battery, comprising:

a voltage sensor responsive to an output voltage of the battery;

a current sensor responsive to a current of the battery;

a secondary current circuit coupled to the current sensor and comprising:

a first transistor and a second transistor coupled in parallel; and

a voltage source circuit to selectively operate the first and second transistors by applying a predetermined constant voltage to each of the first and second transistors; and

a logic unit responsive to the voltage sensor and the current sensor, wherein the logic unit is configured to:

determine a baseline internal resistance of the battery;

calculate an aged internal resistance of the battery based on the current and the output voltage; and

estimate the state of health of the battery based on the baseline internal resistance and the aged internal resistance.

2. The apparatus according to claim 1 , wherein the logic unit is configured to:

determine whether the battery is charging or discharging; and

estimate the state of health of the battery only if the battery is discharging.

3. The apparatus according to claim 2 , further comprising:

a memory device responsive to the logic unit and storing a first battery characteristic indicating a relationship between the voltage and a relative state of charge of the battery; and

wherein the logic unit is configured to determine whether the battery is charging or discharging according to the relative state of charge of the battery, the voltage, and the first battery characteristic.

4. The apparatus according to claim 1 , wherein the logic unit is configured to determine the baseline internal resistance of the battery according to the relative state of charge of the battery.

5. The apparatus according to claim 4 , further comprising:

a memory device responsive to the logic unit and storing a second battery characteristic indicating a relationship between an internal resistance and a relative state of charge of the battery; and

wherein the logic unit is configured to determine the baseline internal resistance of the battery according to the relative state of charge of the battery and the second battery characteristic.

6. The apparatus according to claim 1 , further comprising a temperature sensor coupled to the logic unit and responsive to a temperature of the battery, wherein the logic unit is configured to adjust the aged internal resistance of the battery according to the temperature of the battery.

7. A method for determining a state of health of a battery comprising:

measuring an output voltage of the battery;

measuring a current of the battery;

calculating an aged internal resistance of the battery based on the current and the output voltage;

applying a secondary current to the battery, wherein applying the secondary current comprises applying a voltage to at least one of a first transistor and a second transistor coupled in parallel and maintaining the current through the first transistor and the second transistor at a constant level; and

estimating the state of health of the battery based on the aged internal resistance.

8. The method of claim 7 , further comprising:

determining a relative state of charge of the battery;

determining a baseline internal resistance of the battery based on the relative state of charge of the battery; and

estimating the state of health of the battery based on the calculated aged internal resistance and the baseline internal resistance of the battery.

9. The method of claim 7 , further comprising determining a battery condition, wherein the battery condition is one of a charging state and a discharging state.

10. The method of claim 9 , wherein the current is measured only if the state of charge of the battery is a discharging state.

11. The method of claim 7 , further comprising:

measuring a temperature of the battery; and

adjusting the aged internal resistance based on the temperature.

12. A system for indicating a state of health of a battery, comprising:

a voltage sensor responsive to an output voltage of the battery;

a memory device storing:

predetermined first battery characteristic data indicating a relationship between the voltage and a relative state of charge of the battery;

predetermined second battery characteristic data indicating a relationship between a baseline internal resistance and a relative state of charge of the battery;

a current sensor responsive to a current of the battery;

a secondary current circuit coupled to the current sensor to effect a change in the output voltage; and comprising a first transistor and a second transistor coupled in parallel with each other; and

a logic unit responsive to the voltage sensor, the temperature sensor, and the current sensor and having access to the memory device, wherein the logic unit is configured to:

determine the relative state of charge of the battery;

determine the baseline internal resistance of the battery according to the relative state of charge and the second battery characteristic data;

calculate an aged internal resistance of the battery based on the current and the output voltage; and

estimate the state of health of the battery based on the baseline internal resistance and the aged internal resistance; and

a display unit coupled to the logic unit to display the state of health of the battery, wherein the display unit displays at least one of: a general indicator and a specific indicator related to the state of health of the battery.

13. The system according to claim 12 , wherein the secondary current circuit further comprises a voltage source circuit to selectively operate the first and second transistors by applying a predetermined constant voltage to each of the first and second transistors.

14. The system according to claim 12 , further comprising a temperature sensor coupled to the logic unit and responsive to a temperature of the battery, wherein the logic unit is configured to adjust the aged internal resistance of the battery according to the temperature of the battery.

15. The system according to claim 12 , wherein the second battery characteristic data comprises data describing the internal resistance of the battery as the battery ages.

16. The system according to claim 12 , wherein the logic unit is further configured to:

determine whether the battery is in the charge state or a discharge state according to the output voltage and the relative state of charge of the battery; and

measure the current only if the battery is in the discharge state.